The present invention relates to an adjustable mounting system for a leaning post for a boat having a pair of side-by-side seat posts. The leaning post is provided with a pair of brackets to which hollow cylindrical support posts can be adjustably mounted. The spacing between the support posts can be adjusted to align with the spacing of a pair of conventional hollow cylindrical side-by-side vertical seat posts. A pair of spaced apart parallel slots in the mounting brackets receive bolts that secure the support posts to the mounting bracket and allow the distance between the support posts to be adjusted to align with the distance between the seat posts.